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  Java Practices -> Examine bytecode
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<div class='page-title'>Examine bytecode</div>

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<br>Examination of the bytecode of a class is occasionally useful, in particular
for investigating performance optimizations.
<p>The <tt><a href="http://java.sun.com/j2se/1.5.0/docs/tooldocs/windows/javap.html">javap</a></tt>
tool is included in the JDK for this purpose. Here is an example of its
use, where it shows the effect on bytecode of setting fields explicitly
to their default initial values :
<p><tt>>javap -c -classpath . Quark</tt>
<br><tt>Compiled from Quark.java</tt>
<br><tt>public final class Quark extends java.lang.Object {</tt>
<br><tt>&nbsp;&nbsp;&nbsp; public Quark(java.lang.String,double);</tt>
<br><tt>}</tt>
<p><tt>Method Quark(java.lang.String,double)</tt>
<br><tt>&nbsp;&nbsp; 0 aload_0</tt>
<br><tt>&nbsp;&nbsp; 1 invokespecial #1 &lt;Method java.lang.Object()></tt>
<br><tt>&nbsp;&nbsp; 4 aload_0</tt>
<br><tt>&nbsp;&nbsp; 5 aconst_null</tt>
<br><tt>&nbsp;&nbsp; 6 putfield #2 &lt;Field java.lang.String fName></tt>
<br><tt>&nbsp;&nbsp; 9 aload_0</tt>
<br><tt>&nbsp; 10 dconst_0</tt>
<br><tt>&nbsp; 11 putfield #3 &lt;Field double fMass></tt>
<br><tt>&nbsp; 14 aload_0</tt>
<br><tt>&nbsp; 15 aload_1</tt>
<br><tt>&nbsp; 16 putfield #2 &lt;Field java.lang.String fName></tt>
<br><tt>&nbsp; 19 aload_0</tt>
<br><tt>&nbsp; 20 dload_2</tt>
<br><tt>&nbsp; 21 putfield #3 &lt;Field double fMass></tt>
<br><tt>&nbsp; 24 return</tt>
<p>Here is the source class itself :
<br>
<PRE>

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> Quark {

  <span class='keyword'>public</span> Quark(String aName, <span class='keyword'>double</span> aMass){
    fName = aName;
    fMass = aMass;
  }

  <span class='comment'>// PRIVATE //
</span>
  <span class='comment'>//WITHOUT redundant initialization to default values
</span>  <span class='comment'>//private String fName;
</span>  <span class='comment'>//private double fMass;
</span>
  <span class='comment'>//WITH redundant initialization to default values
</span>  <span class='keyword'>private</span> String fName = <span class='keyword'>null</span>;
  <span class='keyword'>private</span> <span class='keyword'>double</span> fMass = <span class='literal'>0.0d</span>;
} 
</PRE>
<br>
<br>Bytecode instructions are defined by the <a href="http://java.sun.com/docs/books/vmspec/2nd-edition/html/VMSpecTOC.doc.html">JVM
specification</a>.
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction9b86-2.html?Id=14'>Initializing fields to 0-false-null is redundant</a> <br>
 
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